1. Field of the Invention
The present invention relates in general to a method for manufacturing pressure vessels having at both end domes thereof holes of different diameters and preferably used for high pressure combustors of rocket engines, and more particularly to a method for manufacturing such pressure vessels by a filament winding method.
2. Description of the Prior Art
Conventionally, several types of high pressure vessels or high pressure pipes which must efficiently endure high pressure and also have light weight are preferably made of a high strength material such as fiberglass reinforced plastics (FRP), generally obtained by impregnating a base material such as high strength glass fiber strands with a low pressure molding resin such as a thermosetting resin. In order to form the pressure vessels using FRP, there have been proposed several methods, for example, a filament winding method wherein the composite material of FRP obtained from impregnating the glass fiber filament with the thermosetting resin in liquid or gel is continuously spirally wound on a mandrel of a predetermined shape.
FIGS. 1A and 1B show a representative embodiment of the known filament winding method for manufacturing a conventional pressure vessel having opposite holes of equal outer diameters. As shown in the drawings, a winding band 3 obtained from impregnating a plurality of glass fibers with the thermosetting resin is continuously spirally wound on a cylindrical mandrel 1 having a predetermined cylindrical shape integrally formed with domes at both ends. The domes are provided at their peaks with metal bosses 2 each of which has an outer diameter d. Here let the outer diameter of the pressure vessel and the outer diameters of the two bosses 2 be D and d(d=d.sub.1 =d.sub.2), respectively, the winding inclination angle .alpha. of the winding band 3 on the mandrel with respect to the central axis of the mandrel 1 will be described as follows: EQU .alpha.=sin.sup.-1 (d/D) (1)
In winding the band 3 on the mandrel 1 the winding band 3 is repeatedly tightly wound on the cylindrical mandrel 1 such that it is tensioned. In result, the winding passage between two optional points of the winding band 3 on the mandrel 1 always represents a straight line. At the same time, the winding band 3 on the both domes tangentially passes by a point of the outer circumference of the bosses 2 so that it is always perpendicular to the radial direction of the bosses 2, respectively, as depicted in FIG. 1B.
In accordance with the expression (1), it is known that the winding angle .alpha. of the band 3 is proportional to the outer diameter d of the bosses 2 when the diameter D of the mandrel 1 is constant.
Thus, in case that the diameters d.sub.1 and d.sub.2 of the opposite bosses 2 of the mandrel 1 are equal to each other or the diameters d.sub.1 and d.sub.2 are very similar to each other such that the difference therebetween is negligible, the winding angle .alpha. of the band 3 on both domes are equal to each other so that a winding passage of the band 3 between two optional points of the mandrel 1 represents a straight line. Therefore, there will occur no problem in executing the conventional filament winding method for manufacturing the pressure vessels in case of the mandrel 1 including the bosses 2 of same or very similar diameters with or to each other.
However, if it is required to manufacture using the filament winding method a pressure vessel including opposite holes of different diameters, the mandrel 1 must have bosses 2 of different diameters, causing substantial diameter difference which can not be negligible. In this case, the winding angles .alpha. of the band 3 on the opposite domes are different from each other so that there occurs the following problems in executing the conventional filament winding method:
Let the band 3 be wound on the mandrel 1 on the basis of a dome having a boss 2 of a relatively smaller diameter. At the dome having the smaller diameter boss 2, the band 3 is wound on the mandrel 1 at a relatively gentle winding angle so as to maintain the perpendicular relationship with respect to the radial direction of the boss 2 of smaller diameter. Similarly at another dome having a boss 2 of a relatively larger diameter, the winding band 3 tends to be wound on the mandrel 1 at the same gentle winding angle as that of the dome having the smaller diameter boss. However, the winding band 3 has to tangentially pass by the outer circumference of the larger diameter boss 2 at the same time. In result, the winding band 3 on the dome having the larger diameter boss slightly slides due to the tensile force biased thereto, thereby causing the winding angle to vary. The winding band 3 thus tangentially passes by the outer circumference of the larger diameter boss 2 simultaneously with being wound on the mandrel 1 at a relatively steep winding angle.
Hence, the winding band 3 wound on the mandrel 1 at the gentle winding angle slides on the mandrel 1 such that the winding angle of the band 3 with respect to the whole surface of the mandrel 1 varies from the gentle winding angle at the side of the dome having the smaller diameter boss to the steep winding angle at the side of the other dome having the larger diameter boss. The winding passage of the winding band 3 between two optional points on the outer surface of the mandrel 1 is, therefore, not straight but curved.
On the other hand, let the winding band 3 be wound on the mandrel 1 on the basis of the dome having the relatively larger diameter boss 2, the winding band 3 thus tangentially passes by the outer circumference of the larger diameter boss 2 so that the winding angle of the band 3 on the mandrel 1 at the side of dome having the larger diameter boss is relatively steep. In this case, let the winding band 3 be wound without sliding on the mandrel 1 at the side of dome having the smaller diameter boss 2 irrespective of the diameter difference between two bosses 2, the band 3 is thus obliged to pass along a winding passage which is spaced apart from a point of the outer circumference of the smaller diameter boss 2 and also is perpendicular to the radial direction of the boss 2. Therefore at the dome having the smaller diameter boss, there remains a portion on which the winding band 3 is not wound so that it is impossible to provide a desired pressure vessel.
Additionally in case that the winding band 3 radially outwardly slides on the mandrel 1 at the side of the dome having the smaller diameter boss 2, the winding band 3 slips off the outer surface of the mandrel 1 such that it is impossible to wind the winding band 3 on the mandrel 1 as desired. On the contrary, if the winding band 3 radially inwardly slides on the mandrel 1 at the side of the dome having the smaller diameter boss 2, the winding band 3 tangentially contacts with a point of the outer circumference of the smaller diameter boss 2 such that due to the tensile force biased thereto the band 3 runs along a passage which is perpendicular to the radial direction of the smaller diameter boss 2 and has a relatively gentle winding angle than the winding angle at the side of the other dome having the larger diameter boss 2. The winding angle of the band 3 on the whole mandrel 1 thus varies from the gentle winding angle at the dome having the smaller diameter boss to the steep winding angle at the other dome having the larger diameter boss.
In accordance, the winding passage of the winding band 3 between two optional points on the outer surface of the mandrel 1 is not straight but curved. Here, the curvature of the curve of the band 3 on the mandrel 1 is inversely proportional to the diameter difference between two bosses 2, in other words, the winding passage along which the winding band 3 is wound on the mandrel 1 is steeply curved in proportion to the diameter difference between the two bosses 2.
It is well known that as the fiber glass impregnated with the thermosetting resin is wound on the mandrel along a curved winding passage, it has not good structural strength enduring high pressure due to the longer distance between two optional points of the curved winding passage than the straight winding passage. In result, the pressure vessel of which the vessel body comprises the fiber glass wound in the curved winding passage is apt to easily broken by even a low pressure.
As described above, the conventional filament winding method involves no problem in manufacturing a pressure vessel having at both dome peaks thereof the holes of the same diameter with each other, but involves several problems in manufacturing a pressure vessel having at both dome peaks the holes of the different diameters from each other.